1. Field of the Invention
The present invention relates to a rangefinder apparatus for measuring the distance to an object, and, in particular, to an active rangefinder apparatus suitably used in a camera or the like.
2. Related Background Art
In active type rangefinder apparatus used in cameras and the like, an infrared light-emitting diode (IRED) projects a luminous flux toward an object to be measured, the reflected light of thus projected luminous flux is received by a position sensitive detector (PSD), a signal outputted from the PSD is arithmetically processed by a signal processing circuit and an arithmetic circuit and then is outputted as distance information, and the distance to the object is detected by a CPU. In general, since errors may occur when the distance is measured upon a single light-projecting operation, light is projected a plurality of times so as to obtain a plurality of distance information items, which are then integrated by an integrating circuit and averaged.
As such an active type rangefinder apparatus, one shown in FIG. 1 has conventionally been known. FIG. 1 is a configurational view of the rangefinder apparatus in accordance with first prior art.
In the rangefinder apparatus shown in this drawing, under the control of a CPU 110, a driver 112 drives an IRED 114 so as to make it output infrared light, which is then projected through a light-projecting lens 201 to an object to be measured. The infrared light reflected by the object is collected by a PSD 116 by way of a light-receiving lens 202, and the PSD 116 outputs two signals I.sub.1 and I.sub.2 according to the position at which the reflected light of the infrared light is received. A first signal processing circuit 118 eliminates a steady-state light component contained in the signal I.sub.1 which becomes a noise, whereas a second signal processing circuit 120 eliminates a steady-state light component contained in the signal I.sub.2 which becomes a noise.
According to the signals I.sub.1 and I.sub.2 from which the steady-state light components have been eliminated, an arithmetic circuit 132 determines an output ratio (I.sub.1 /(I.sub.1 +I.sub.2)) by an arithmetic operation, and outputs an output ratio signal corresponding to the distance to the object. An integrating circuit 134 integrates the output ratio signals thus outputted from the arithmetic circuit 132 a plurality of times, thereby improving the S/N ratio. The signal outputted from this integrating circuit 134 (hereinafter referred to as "AF signal") corresponds to the distance to the object. Then, according to the AF signal outputted from the integrating circuit 134, the CPU 110 determines a distance signal by carrying out a predetermined arithmetic operation, and controls a lens driving circuit 136 according to this distance signal, so as to move a lens 138 to an in-focus position.
FIG. 2 is a graph showing the relationship between the AF signal outputted from the integrating circuit 134 in this first prior art and the distance to the object. In this graph, the abscissa indicates the reciprocal (1/L) of the distance L to the object, whereas the ordinate indicates the output ratio (I.sub.1 /(I.sub.1 +I.sub.2)), i.e., AF signal. As shown in this graph, the output ratio has substantially a linear relationship with respect to the reciprocal (1/L) of the distance L at a certain distance L.sub.4 or less, such that the output ratio decreases as the distance L is longer (1/L is smaller). At the distance L.sub.4 or greater, by contrast, the influence of the noise component increases as the distance L is greater. Letting I.sub.n (I.sub.n 0) be the noise component, the output ratio is (I.sub.1 +I.sub.n)/(I.sub.1 +I.sub.n +I.sub.2 +I.sub.n), whereby the output ratio would shift so as to increase (toward an output ratio of 50%) at the distance L.sub.4 or greater. Also, since I.sub.n occurs randomly, it becomes unstable depending on the distance measuring condition. It is due to the fact that, as the distance L increases, the intensity of reflected light received by the PSD 116 decreases, whereby the noise component I.sub.n becomes relatively greater. If such a phenomenon occurs, the distance to the object L cannot be determined uniquely from the output ratio.
Therefore, as shown in FIG. 3, a clamping circuit 13 which outputs a clamp signal I.sub.c if the far-side signal I.sub.2 outputted from the second signal processing circuit 120 is lower than the clamp signal I.sub.c is disposed between the second signal processing circuit 120 and the arithmetic circuit 132. Even in this case, however, as shown in FIG. 4 which will be explained later, the distance output is fixed at a certain constant distance on the far distance side, whereby the deviation from the designed value may become greater.
Hence, as a rangefinder apparatus overcoming such a problem, the following one has been known. FIG. 5 is a configurational view of the rangefinder apparatus in accordance with the second prior art. This drawings shows the light-receiving side alone. In the rangefinder apparatus shown in this drawing, the signals I.sub.1 and I.sub.2 outputted from a PSD 140, having their steady-state light components eliminated therefrom by their respective steady-state light eliminating circuits 142 and 144, are inputted to both of arithmetic circuits 146 and 148. According to the signals I.sub.1 and I.sub.2 with no steady-state light components, the arithmetic circuit 146 carries out an arithmetic operation of I.sub.1 /(I.sub.1 +I.sub.2), so as to determine the output ratio, and an integrating circuit 152 integrates this output ratio. On the other hand, an arithmetic circuit 148 carries out an arithmetic operation of I.sub.1 +I.sub.2, so as to determine the quantity of light, and an integrating circuit 152 integrates this quantity of light. Then, a selecting unit 160 selects one of the output ratio and the quantity of light, and determines, based thereon, the distance to the object to be measured. Here, the selecting unit 160 is a processing operation in a CPU.
FIG. 6 is a configurational view of the rangefinder apparatus in accordance with the third prior art. This drawings shows the light-receiving side alone. In the rangefinder apparatus shown in this drawing, the signals I.sub.1 and I.sub.2 outputted from a PSD 170, having their steady-state light components eliminated therefrom by their respective steady-state light eliminating circuits 172 and 174, are inputted to one end of a switch 176. The switch 176 is controlled by a CPU, and inputs one of the outputs of the steady-state light eliminating circuits 172 and 174 to an integrating circuit 178. The integrating circuit 178 integrates one of the inputted signals I.sub.1 and I.sub.2, whereas an arithmetic unit 180 carries out an arithmetic operation of I.sub.1 /(I.sub.1 +I.sub.2) according to the result of integration, so as to determine the output ratio. On the other hand, an arithmetic unit 182 carries out an arithmetic operation of I.sub.1 +I.sub.2, so as to determine the quantity of light. Then, a selecting unit 184 selects one of the output ratio and the quantity of light, and determines, based thereon, the distance to the object to be measured. Here, the arithmetic units 180, 182 and the selecting unit 184 are processing operations in the CPU.
In both of the rangefinder apparatus in accordance with the second and third prior art examples (FIGS. 5 and 6), the distance L to the object to be measured is determined according to the output ratio (I.sub.1 /(I.sub.1 +I.sub.2)) and the light quantity (I.sub.1 +I.sub.2) when the distance L is shorter and longer, respectively. Such a configuration makes it possible to uniquely determine the distance L.